1. Field of the Invention
The invention relates to addition-crosslinkable silicone compositions, to processes for preparing them, to the silicone elastomers obtainable therefrom, and to their production and their use.
2. Background Art
It is known that addition of polydiorganosiloxanes having lateral alkenyl groups to alkenyldiorganosiloxy-terminated polydiorganosiloxanes having a viscosity of up to 200,000 mPas gives addition-crosslinkable liquid silicone rubbers, known as LSR compositions, which, after crosslinking, provide silicone elastomers having improved mechanical properties. This applies particularly to tear propagation resistance in accordance with ASTM D 624 B. To improve the mechanical properties, in particular the tear propagation resistance, polydiorganosiloxanes having terminal and lateral alkenyl groups or low-viscosity alkenyldiorganosiloxy-terminated polydiorganosiloxanes are added to alkenyldiorganosiloxy-terminated polydiorganosiloxanes.
EP 0 305 073 A2 describes the improvement of the mechanical properties, in particular the tear propagation resistance, by addition of a polydiorganosiloxane which bears vinyl groups both at the end of the chain and in the chain and has a viscosity of 100-200,000 mPas, to a vinyl-terminated polydiorganosiloxane having a viscosity of from 20 000 to 200,000 mPas. Silicone elastomers having a tear propagation resistance of about 40 N/mm (ASTM D624) and an elongation at break of a maximum of 580% at a hardness of about 50 Shore A can be obtained from this polymer combination.
EP 0 695 787 A2 describes the improvement of tear propagation resistance by means of a low molecular weight vinyl-terminated polydiorganosiloxane having a viscosity of from 1.0 to <50 mPas. However, the addition of vinyl-terminated polydiorganosiloxanes having such a low viscosity results in a reduction in the elongation at break.